Vacuum die casting of molten metals



A ril 20, 1965 F. c. BENNETT VACUUM DIE CASTING 0F MOLTEN METALS Filed Feb. 14, 1962 INVENTOR. Foa/er 61 Ben/761V United States Patent 3,178,782 Patented Apr. 20, 1965 tice 3,178,782 I VACUUM DEE CASTING F MGLTEN METALS Foster C. Bennett, Midland, Mich, assignor to The Dow Chemical Company, Midland, Mich, a corporation of Delaware Filed Feb. 14, 1962, Ser. No. 173,287 1 Claim. (U. 2268) This invention relates to the die casting of molten metals and more particularly is concerned with an improvement in the apparatus and method used for vacuum die casting molten metals.

The term vacuum die casting or vacuum as used herein and as is understood in the die casting art means a die casting process wherein subatmospheric pressures are applied to the shot well, runner and mold cavity of a die casting machine thereby serving to pull a molten metal shot into the mold. In actual operation the vacuum ranges from only a little below atmospheric, i.e. from a maximum of about 27 or 28 inches mercury absolute, to about 1 to 2 inches mercury absolute or less.

In conventional vacuum die casting apparatus employing a plunger, the metal charge is pulled into the mold simultaneously as it is being evacuated while the injection piston, i.e. plunger, enters the cavity at some later time. This method suifers from two major defects: (1) Because of metal entry into the mold during evacuation, the amount of actual vacuum in the cavity at any given time is not known nor can it readily be controlled; and, (2) there is no certainty that the die cut-oli is being actuated at the instant the die is filled with the proper amount of metal. Both overfilling the die with its accompanying dangerous molten metal splashing and flashing and underfilling which results in poor or incomplete castings are common occurrences in the method as practiced conventionally. Additionally, many times in the process as now practiced, lubricant flashes out of control. These problems also exist in other vacuum die casting apparatus which use only high vacuum to draw molten metal into the shot well and inject it into the die cavity.

It is a principal object of the present invention to provide an apparatus and method whereby there is an assured level of vacuum in the die cavity or" a vacuum die casting apparatus before any metal is metered therein.

it is another object of the present invention to provide means for preventing over-filling the shot well of a vacuum die casting apparatus.

It is a further object of the present invention to provide an apparatus and method in which lubricant flashing and vacuum loss in the die cavity can be detected prior to introducing molten metal therein and can be corrected or compensated as desired prior to introducing metal into the cavity.

It is an additional object of the present invention to provide a system where good vacuum conditions consistently can be obtained and maintained during operation of a vacuum die casting apparatus.

It is another object of the present invention to provide a large bore tube for use in feeding molten metal into the die cavity of a vacuum die casting machine whereby proper operating temperatures readily are retained and low metal velocities are employed thereby giving substantially no metal splashing during the die casting operation.

It is a further object of the present invention to provide an apparatus which controls the rate and quantity of metal flow into the shot Well of a vacuum die casting apparatus.

It is another object of the present invention to provide an improved apparatus and method for transferring predetermined quantities of molten magnesium from a holding pot to the shot well of a die casting machine.

These and other objects and advantages will become apparent from the detailed description presented hereinafter when read in accordance with the accompanying drawings.

In general, the present invention comprises a valve mounted on the intake of a large bore suction tube of a vacuum die casting machine, which tube is immersed in a pot of molten metal. This valve is operated in a predetermined controlled cycle in cooperation with the action of the die casting machine.

FIGURE 1 is a schematic view, partly in section of one embodiment of the valve assembly of the present invention mounted on a conventional plunger type vacuum die casting machine.

FIGURE 2 is a fragmentary view, partly in section, showing a second embodiment of the valve assembly of the present invention mounted on a similar vacuum die casting apparatus.

In the embodiment depicted in FIGURE 1, valve assembly 11 is positioned so as to cooperate with the open end 12 of suction tube 13 of vacuum die casting apparatus 14. The other end of suction tube 13 is restricted into an orifice 5 at its junction with the cylindrical shot well 16 of the die casting machine 14. The suction tube 13 is rigidly aflixed to the side wall 17 of shot well 16 and establishes communication into shot well 15. Ordinarily the main portion of this suction tube 13 will be of a large bore, i.e. having an interior cross-sectional diameter of from about /2 inch to about 3 inches. Preferably, a combination gasket and gas pressure seal 18 of thermally insulating material, e.g. metal covered asbestos, is positioned between the suction tube 13 and side wall 17 of shot well 16 at this junction.

Conveniently, the suction tube 13 and gasket 13 can be held tightly in place against the side wall 17 of shot well 16 by means of a C-clamp 19 so as to form a liquid and pressure tight seal between these members. As depicted, the lower arm 28 of C-clamp 19 is divided and slides around the outside suction tube 13 below a projecting ring 21 integral to tube 13. The stem 22 of the clamp also is divided so as to slide over the outside of shot Well 16. The upper arm 23 of clamp 19 is fitted with a threaded opening 24 near its end 25. A. thumb screw 26 is screwed through opening 24 and is tightened against the wall 17 of shot well 16 whereby the lower clamp arm 20 exerts pressure against ring 21 thereby holding the suction tube 13 and gasket 18 tightly against the wall 17 of shot well 16.

Other means of holding the sucetion tube and gasket to the shot well as known to one skilled in the art can be employed. Piston 27 slidably moves within and along the bore of the shot well 17 and conventionally is positioned and held in place by a vacuum cap 28 affixed to the end of the shot well 16. A pressure-tight seal between the cap 28 and both the shot well 16 and piston 27 is assured through use of conventional G-rings 29 or other seals.

The valve assembly 11 comprises a valve head 30 mating with a valve seat 31 in the open end 12 of suction tube 13. The valve head 30 is mounted on a valve stem 32. Valve stem 32 in turn is attached to the bottom cross rod 33 of yoke assembly 34. The yoke assembly 34 is attached by means of a rod 35 affixed to the upper yoke cross rod 36 of yoke assembly 34 to an air-driven reciprocating driving means 37 In the depicted embodiment, the valve head 30 is shown to bee frustum and is fastened onto the valve stem 32 by means of a nut 38 mating with the threaded end portion 39 of the valve stem 32. However, this valve head 30 can be of diverse shapes; e.g. the valve head can be round-bottomed. Also, it can be welded, press fitted or otherwise fastened onto the valve stem 32.

The angle of taper of the valve head 30 and curvature of valve seat 31 should be great enough to avoid wedging of these members upon impact. Preferably, this angle, as measured from the longitudinal axis of the valve stem to the tapered surface, at a minimum is about For those valve assemblies wherein the valve head is curved and valve seat is tapered, this same angular relationship holds true.

In the second illustrated valve embodiment, the posi:

In operating the improved apparatus of the present invention, the mold, runner and shot well of a vacuum die casting machine are evacuated in a conventional manner, the valve at the end of the suction tube immersed in a pot of molten metal 41 to be metered being closed and the injection piston being adjacent the vacuum cap at the end of the shot well. As the predetermined vacuum is reached in the machine, the valve at the end of the suction tube is opened for a timed interval whereupon a given quantity of metal is drawn into the shot well. The actual amount of metal transferred into the shot Well for a particular casting operation can be accurately controlled by varying the mount of vacuum present in the apparatus and/ or changing the time intervalthat the valve is open. When the. desired amount of metal has been drawn into the shot well, the valve is closed, and, in a plunger type 7 machine, the plunger is activated thereby injecting the tion of the valve head on the stem 32 is reversed; i.e.'

the larger diameter base of the frustum is within the suction tube 13 and the valve action is reversed.

Preferably the valve is activated by positive mechanical means rather than by gravity or spring motion. This is desirable for two reasons: (1) in operation the time cycle relationship for valve operation must be closely controlled and (2) ordinary spring-activated check valves do not give positive shut-off with readily oxidizable molten metals, e.g. magnesium and aluminum. Conveniently the valve assembly is activated through an automatic timer, e.g. an interval timer, thereby assuring precise control of the apparatus and automatic transfer of reproducibly constant predetermined quantities of metal to the shot well;

As shown in the illustrative embodiments, the valve is activated by an. air driven piston-cylinder, device 37 through a yoke and rod arrangement. It is apparent that the valve assembly can be operated by any of a wide variety of devices producing reasonably reciprocating linear motion. The depicted air driven device is one preferred means since it is simple, easily and accurately con- I,

cise, controlled, integrated schedule or program. Likewise, valves differing in construction and design from those described herein can be used as is apparent to one skilled in the art.

The size of the restraining orifice 15 which forms the l port whereby communication is established between suction tube 13 and shot well 16 can be of a given predetermined size so as to give a metal flow into the shot well of a desired controlled velocity.

The materials of construction to be employed for the valve assembly are those that have the requisite strength, stability of temperature and erosion and corrosion resistance to stand up in the presence of the molten metal being moved. With magnesium (i.e. magnesium and magnesium based alloys containing at least about 70 percent by weight magnesium) chrome steels such as 430 stainless, for example, are satisfactory for the surfaces contacting the molten metal. For molten aluminum (i.e. aluminum and aluminum basedalloys containing at least about 70 percent by weight aluminum) the valve assembly can be fabricated from steel having a refractory coating with hearing surfaces being coated with or constructed from titanium diboride, zirconium diboride or other construction material inert to molten aluminum. I

Either the valve head surface, valve seat surface or.

both can be given a Stellite coating, e.g. Stellite No. 1,'

The plunger is then returned to its The apparatus now is ready for anmetal into the die. original position. other cycle.

With a die casting machine employing high vacuum alone and no plunger action, the valve operation is carried out in a similar time cycle, this cycle again being coordinated with die casting machine operation. 7 7 Each cycle may be started manually when a die casting machine is cleared and is ready for another metal shot or it may be started automatically with suitable safeguards to prevent premature, late or double metal discharge.

With a valve arrangement as shown in FIGURE 2, during operation as the valve is shifted from open to closed position there is gained the additional advantage that molten metal is sucked-back slightly thereby substantially reducing the possibility that any metal will enter the shot Well behind the moving piston. This suckback by the valve is small enough so the suction tube 13 remainssubstantially full of molten metal between shots whereby good temperature control and metal cleanliness are maintained.

The utility of the instant improved vacuum die casting machine was demonstrated in an operation whereby reproducibly controlled amounts of molten ASTM designated AZ91B magnesium alloy (nominal composition 9 percent aluminum, O.7 percent zinc, 0.15 percent manganese, balance magnesium) were transferred from a holding pot to the shot well of a vacuumjdie casting machine. For this operation, a 430 stainless steel valve assembly and suction tube having a valve seat at its lower end were inin the appended claim.

I claim:

In a vacuum die casting machine for molten metals the improvement in combination therewith which comprises; providing a large bore suction tube having an interior cross-sectional diameter of from about /2 inch to about 3 inches rigidly attached by one end to the cylindrical side wall of the shot well of a die casting machine and establishing communication into said shot well, said suction tube being restricted into an orifice at its junction with said shot well, a combination gasket and gas pressure seal positioned between the suction tube and the side wall of said shot well at the junction of these members, said suction tube having a valve seat at its open end opposite the of said yoke assembly to a positive mechanical drive means, said mechanical drive means connected to and activated by an automatic timer thereby opening and closing said valve in accordance with a predetermined, planned program thereby to introduce reproducibly constant predetermined quantities of molten metal to said 5 shot well.

References Cited by the Examiner UNITED STATES PATENTS Galloway 2273 Junghans 2279 XR Short 2279 XR Federman et a1 2273 Morgenstern 2273 Sylvester 22-69 Adams 2273 XR MICHAEL V. BRINDISI, Primary Examiner. 2,137,764 11/38 Wagner 2268 RAY K. WINDHAM, Examiner. 

